Censorship on China's top social messaging app WeChat goes beyond the country's borders, according to a report released on Thursday, with messages containing sensitive terms that are sent from overseas to mainland-linked accounts being blocked.

China has stepped up efforts to remove non-sanctioned online content in the past year and increase surveillance measures, garnering support from top Chinese tech firms including Alibaba Holdings Ltd and Tencent Holdings Ltd, which owns WeChat.

"Tencent respects and complies to local laws and regulations in countries we operate in," a Tencent spokeswoman told Reuters, declining to comment on the censorship features directly.

Messages from overseas accounts containing terms deemed sensitive by the Chinese government, such as references to the banned Falun Gong spiritual movement, do not appear on the service after being sent, if sent to or from an account linked to a Chinese phone number, a report released by Toronto-based Citizen Lab showed on Thursday.

The issue particularly affected group chats including three or more users and continued to affect users who later switched to a foreign phone number, according to the researchers.

"(WeChat) features are based on account registration and censorship is another feature they can turn on or off depending on what kind of account you have," Masashi Crete-Nishihata, one of the researchers on the project, told Reuters.

"Whether the motive is intentional or accidental, the outcome is the same: mainland China WeChat users face censorship regardless of where they are in the world," he said.

China formalized strict real-name authentication regulations for tech companies in 2016, requiring users on social media and e-commerce platforms to link their real name to a phone number that can be tracked.

Authorities cite cyber terrorism risks and the preservation of social stability as reasons for the heightened surveillance.

WeChat has more than 800 million monthly active users who spend an average of 40 minutes a day using the app, which also includes payment, ride-hailing and gaming features.

In a test conducted by Reuters on Thursday, users based overseas who had registered with a foreign phone number were unable to send sensitive phrases to a group chat containing a China-registered user.

(Reporting by Cate Cadell; Editing by Robert Birsel)